1. Field of the Invention
The present invention relates to adjustable apertures, and more particularly, to adjustable apertures suitable for use in controlling transverse modes in lasers.
2. Description of Related Art
A variety of applications exists for adjustable apertures by which the amount of light that passes through a given passage can be regulated. For instance, photographic systems typically use irises that consist of a plurality of overlapping leaves that form an aperture around their inner perimeter. The position of the leaves is controlled in order to adjust the size of the aperture.
In lasers, apertures are often used to suppress transverse modes of laser oscillation. A given laser cavity can support a number of transverse modes so that a number of different wavelengths and geometries of laser oscillation are set up within the cavity. In order to ensure a uniform beam at the center of the cavity, all transverse modes except a preferred mode designated TEM.sub.00 must be suppressed. This suppression can be accomplished by placing an aperture within the laser cavity that blocks oscillation of laser light in the geometry required to set up the desired transverse mode.
Many laser mediums will lase at a variety of wavelengths. The cross-sectional size of the transverse modes in those lasers will vary depending on the wavelength of light desired to be supported. For shorter wavelengths the mode size of the laser beam is smaller, requiring a small aperture in order to suppress the high order transverse modes. For longer wavelengths, the transverse modes have larger cross-sectional sizes. Therefore, a larger aperture is required to allow the desired mode to survive while suppressing the unwanted higher order transverse modes.
Prior art systems providing adjustable mode control apertures in lasers have used a wheel having a plurality of pre-cut apertures. The wheel is turned so that the desired aperture is placed in alignment with the laser beam. If the wavelength to be produced by the laser is changed, the wheel is turned to align a different aperture with the beam. Obviously an iris-type aperture, common in photographic systems, could be used as well to control transverse modes of lasers.
The disadvantages of using the wheel having a plurality of pre-cut apertures, arise from difficulty in fabricating the wheel. Each aperture must be precisely aligned, with the center of each aperture being an equal distance from the center of the wheel, so that as the wheel turns and a new aperture becomes aligned with the beam, the aperture will remain centered on the beam. Due to the precision required in manufacturing, the aperture wheel becomes costly. Further, because of the precise alignment required to mount the wheel, the reliability of the aperture to produce high quality mode suppression is reduced.
Iris-type apertures are difficult to manufacture in a manner that provides precise control of the desired aperture size for mode suppression in lasers. Further, they involve a large number of pieces that must be assembled in precision manufacturing.
Accordingly, it is desirable to provide an aperture that is continuously adjustable, easy to manufacture and that can be adjusted with a high degree of reliability. Such an aperture would be particularly suited to mode control in lasers.